The WSHumanTaskHandler (and its brother when using persistence, CommandBasedWSHumanTaskHandler) indeed do not copy the fault data. It wouldn't be difficult however to copy this data as a new "Fault" result parameter of the work item. Would this be sufficient for your case?
If so, could you open an issue on the jBPM JIRA and attach this information. If you could give a try at changing the WSHumanTaskHandler implementation to support this, and attach the improvements, that would even be better
So if I can't get Fault Data, how can I detect failed tasks in my BPMN? I want to use Diverge Gateway according to HumanTask execution results (complete or fail)